GB1102792A - Alternator tachometer - Google Patents

Abstract

1,102,792. Measuring speed of internal combustion engines, electrically. STEWARTWARNER CORPORATION. Feb.17, 1965 [Feb.17, 1964], No.6871/65. Heading G1N. A tachometer (e.g. for an automobile engine) comprises a saturable transformer 72 (Fig. 2) the primary 74 of which is connected between an A. C . potential source (terminal 64), from an alternator 42 driven by the engine, and a D.C. bias potential at end 74b, and a meter 70 is connected to measure the time average secondary current as a function of speed. The alternator 42 charges a storage battery 50 through a three phase bridge rectifier network. One end of the primary 74 is connected via a resistor 78a to a positive D.C. potential at 58 and to earth via a resistor 78b so that the potential at terminal 64 fluctuates between earth potential and a positive potential approximating the voltage at 58 (Fig. 3a). In operation, with the transformer core saturated in say a negative direction the primary current wave (Fig.3b) follows the increasing voltage (Fig. 3a) and increases until at point X it causes the core flux density to drop below saturation level and the inductance of the winding therefore increases causing the current to level off. The core flux changes direction until at point Y it reaches saturation in the opposite or positive direction, where at the inductance being negligible the current again increases until it attains a level determined by the limiting resistors 78a, 78b. When the voltage at terminal 64 swings from earth potential in a positive direction relative to the voltage at the opposite end of the primary, the core is driven to saturation in one direction, and when the voltage at 64 swings in a negative direction the primary current reverses and the respective saturation points of the core are indicated at X' and Y' on Fig. 3b. The voltage induced in the secondary winding is shown in Fig. 3c, and follows the primary voltage during the periods X to Y and X' to Y' &c., when the core is not saturated. The secondary voltage is thus a function of the primary current and of the saturation time. The current output of the secondary across a portion of a potentiometer 91 is shown (Fig. 3d) and the average voltage drop thereacross is a function of the frequency of the pulses. By virtue of its mechanical inertia the meter 70 measures average current through it and is calibrated to indicate speed. An NTC resistor 98 may be provided in the secondary circuit to provide temperature compensation.